Soybean Diseases are Present; Is Fungicide Application Warranted?

Nathan Kleczewski, Extension Specialist – Plant Pathology;

Fungicide use in soybeans is a somewhat controversial subject in Delaware. Overall, Delaware soybeans do not suffer to a significant degree from fungal diseases such as white mold or even Frogeye leaf spot. More commonly, we see brown spot and downy mildew on our beans, especially this time of year. Why are these not typically diseases of concern in soybean production?

Downy Mildew
Downy mildew (DM) is a disease that prefers cooler temperatures and persistent humid conditions. On foliage, DM appears as small light green to yellow flecks when viewed from the top (Figure 1). When the leaf is turned over you will see white to grey fuzz under the location of the discolored lesions. Hot, dry weather stops this pathogen dead in its tracks. Often growers see a slight flush in downy mildew right around canopy closure. Then, the disease stops as temperatures increase and we hit summer full on. Downy mildew on soybeans is not a concern as it is in cucurbits. Don’t worry about soybean DM.


Figure 1. Top view showing a leaflet with typical symptoms of downy mildew. When the leaf is flipped over, you will see white to grey fuzz immediately underneath the discolored spots.

Septoria Brown Spot
Septoria brown spot is a soybean-residue borne disease that also requires very persistent, wet conditions. This often means that the disease is restricted to the lower canopy, which is not contributing much to yields later in the season. Lesions often start as small black to brown spots with yellow halos. Over time the leaf will turn yellow and look as if someone splashed it with black paint (Figure 2). Most varieties are very tolerant to brown spot and the disease often has little to no impact on yield except if an extremely susceptible variety is planted. Defoliation upwards of 25% is required before any noticeable yield loss occurs. Rarely are fungicides needed to suppress Septoria brown spot.


Figure 2. Septoria brown spot is often restricted to the lower canopy. Small, black to brown lesions form on green tissue. Over time, the affected foliage may turn yellow and fall from the plant.

Frogeye Leaf Spot
That leaves us with Frogeye leaf spot. This disease is a big problem in the South and we have seen an occasional field with a fair amount of disease. Infection typically occurs after flowering and starts as small brown lesions that expand to form irregularly shaped, grey/tan blotches with purple/red margins. When flipped over, lesions will have a fuzzy grey mass at the center (Figure 3). Stems can also be infected, but this is much less frequent. The pathogen is much better at infecting young, developing foliage, so often plants appear to have “layers” of lesions in the canopy that correspond to periods of persistent wet weather during plant growth. If you do have significant (much more than a lesion here and there) levels of Frogeye in a field at R1, an application of a premix or triazole-based fungicide at R2/3 may provide some benefit if wet, humid conditions are likely to persist in the coming days or you plan on heavily irrigating your soybeans.


Figure 3. The underside of a leaf with symptoms of Frogeye leaf spot. Note the purple margins and blotchy appearance. This can be confused with herbicide injury. Herbicide injury will be more uniform in a field when compared to plants suffering from Frogeye leaf spot.

Stem Canker
Nancy has seen some samples with symptoms characteristic of stem canker arrive at the clinic over the past week. Stem canker is a fairly common disease in our region, particularly under persistent wet conditions like we have had over the previous few weeks. What does stem canker look like? Often you will notice areas of dead plants with dead of dying leaves in the field. The first symptoms of stem canker are small red/brown lesions on the bases of petioles or stems. Over time lesions expand to form corky, sunken, brown/red cankers that gradually expand the length of the stem. Cankers girdle the stem, preventing movement of water and nutrients throughout the plant, eventually resulting in the death of plant parts above the canker. Leaves often develop interveinal necrosis due to a toxin produced by the fungus. Foliar symptoms are not a diagnostic feature of this disease. If you are lucky you may see small, black, pinhead-sized dots on the lesions. There is no within-season management after the disease has appeared, but the disease can be minimized in subsequent years by selecting a resistant variety, rotation to a non-host such as corn, using a balanced nutrient program, and managing residue where appropriate.


Figure 4. A soybean stem with symptoms of stem canker.